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2020Q1(b)(ii)Absorption by Roots – The Processes Involved
DefinitionModerate1 mark
1(b)

Explain the following terms:

Diffusion

Topic: Diffusion

Previous-year question practice database

Answer

Exam answer

DIFFUSION is the movement of molecules of a gas, liquid or dissolved substance from a region of HIGHER concentration to a region of LOWER concentration, until they are evenly distributed. It requires no energy and no membrane.

Explanation

Two elements earn the mark: movement DOWN the concentration gradient, and the absence of any energy requirement.

DISTINGUISH IT from the two processes it is usually asked alongside: OSMOSIS — also passive and down a gradient, but it concerns the SOLVENT (water) only and REQUIRES a selectively permeable membrane ACTIVE TRANSPORT — moves substances AGAINST the gradient and therefore REQUIRES energy from ATP

Diffusion is how gases are exchanged in the alveoli of the lungs and through the stomata of a leaf — which is why it appears in several chapters.

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